In the papermaking art, an aqueous suspension containing cellulosic fibres, and optional fillers and additives, referred to as stock, is fed into a headbox which ejects the stock onto a forming wire. Water is drained from the stock through the forming wire so that a wet web of paper is formed on the wire, and the paper web is further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are conventionally introduced into the stock in order to facilitate drainage and to increase adsorption of fine particles onto the cellulosic fibres so that they are retained with the fibres on the wire.
Silica-based particles are widely used as drainage and retention aids in combination with charged organic polymers like anionic and cationic acrylamide-based polymers and cationic and amphoteric starches. Such additive systems are disclosed in U.S. Pat. Nos. 4,388,150; 4,961,825; 4,980,025; 5,368,833; 5,603,805; 5,607,552; 5,858,174; and 6,103,064. These systems are among the most efficient drainage and retention aids now in use.
Silica-based particles suitable for use as drainage and retention aids are normally supplied in the form of aqueous colloidal dispersions, so-called sols. Such commercially used silica-based sols usually have a silica content of about 7-15% by weight and contain particles with a specific surface area of at least 300 m2/g. Sols of silica-based particles with higher specific surface areas are usually more dilute to improve storage stability and avoid gel formation.
It would be advantageous to be able to provide silica-based sols with further improved drainage and retention performance and even better stability. It would also be advantageous to be able to provide a process for preparing silica-based sols exhibiting improved drainage, retention and stability properties. It would also be advantageous to be able to provide a papermaking process with improved drainage and retention.